they are collections for sure, i have over 300 pairs in my 16mm size mainly because im a manufacturer but also through trading with other manufacturers,
i like to keep some 14mm and 15mm around when i leave my plugs out too long
i also like to keep some SF and some DF for the same reason
and i have heard lots of people keep some of their smaller jewelry so if/when they end up going smaller they still have jewerly.
and part of it is getting to wear some of the unique designs that are only offered at certain sizes.